The present invention relates in general to video projection systems, and more particularly to an apparatus for collecting and sensing light in projectors for use with constant intensity modes of projector operation.
Video projectors require special lamps that are designed to project images of sufficiently high brightness levels to be discerned by the human eye. It is well known that optical brightness of projection lamps, and consequently the images projected thereby, diminishes with operational time and/or age of the lamps.
In order for a projector to maintain a constant brightness level over time, it is normally required to boost the lamp power by small increments in order to offset the diminishing lamp brightness. Automatic methods of increasing ballast power are known, which can be open loop systems or closed loop systems.
Open loop system designs of constant brightness lamp operation require an applied correction factor (of lamp power) that slowly increases the lamp power at a specified rate. One problem with such systems arises when the applied correction factor does not ideally match the brightness decay of a given lamp, with the result that the brightness level varies with time.
Closed loop system designs of constant brightness typically provide better results than open loop systems. Closed loop systems measure the lamp brightness in real-time and adjust the lamp power accordingly through software control. A typical closed loop system requires a light sensing instrument, a method of light measurement, and an electrical interface to the lamp ballast and control software.
In such closed loop systems, the method of light sampling and measurement of on-screen brightness has presented great difficulties.
Other prior art approaches to lamp brightness compensation are exemplified in the patent literature, as discussed below.
U.S. Pat. No. 3,833,297 to Swartz discloses a projector for use with film or transparencies. The projector automatically controls the projected brightness in response to the exposure of the film or to the density of the transparency. Sampling of the light is unobtrusive as only the light that is not transmitted through a focussing lens is sampled. An appropriate electrical circuit for automatic control of the brightness is also disclosed.
U.S. Pat. No. 4,842,404 to Duda discloses a laser beam power monitor having two sampling ports. Two wedge-shaped beam splitters perform obtrusive light sampling by reflecting portions of the laser beam into the sampling ports for detection by photodetectors. The light detected can be monitored for measurement or control purposes.
U.S. Pat. No. 5,537,203 to Carr discloses a diffusely reflecting polymeric material for an integrating sphere. Use of the integrating sphere for measurement of diffuse transmittance of, for example, scattering liquids, translucent films, topical creams and biological and other specimens is contemplated.
U.S. Pat. No. 5,650,843 to Moberg et al. discloses an apparatus for sampling of light, and feedback control of the light intensity from a light source in a photographic film scanner. Specifically, the system includes a light integrating cavity having a light input port, a slot for emitting integrated light and a feedback port for providing a sample of the integrated light. A fiber optic cable is disposed at the feedback port and receives integrated light from a small radiating surface area within the cavity.
The present invention provides a new method and mechanism of light collection that accurately represents and compensates for lamp brightness decay to maintain screen brightness. More particularly, a secondary mirror is placed behind a primary cold mirror. The primary cold mirror removes UV and IR radiation from illumination. The secondary mirror functions to reflect leaking visible light for collection and constant brightness control. The mechanism is used internally within the projector and is unobtrusive to the light being projected. The invention relies on the fact that the primary cold mirror is positioned directly in the incident light path, and that a small percentage of visible light (normally  less than 4% leakage) is transmitted through the primary cold mirror.
There are two main aspects of the invention. Firstly, the secondary mirror is used to unobtrusively sample the main light beam. Secondly, an integrator box is used to uniformly sample the resultant unfocussed and extremely non-uniform light beam. The integrator box is preferably constructed to trap and attenuate infrared radiation and reflect visible light to sample the on-screen brightness.
While the use of an xe2x80x9cintegrating spherexe2x80x9d for sampling light is known from the prior art set forth above, none of the prior art discloses a xe2x80x9csecondxe2x80x9d mirror element which is used to unobtrusively sample the main light beam. Nor does the prior art disclose the use of an integrator box to trap and attenuate infrared radiation and reflect visible light to sample the on-screen brightness.